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Air flow rate between rafters in a vented attic

KYLE WINSTON BENTLEY | Posted in Energy Efficiency and Durability on

I was wondering about something today, and I couldn’t really come across any research on the subject, though I haven’t really been looking that hard at the same time.

Does anyone know if there’s data out there on the volume flow rate of air from soffit vent to ridge vent in unconditioned, ventilated attics?  Bonus case if it’s the flow rate near the surface of the sheathing between rafters.  I had assumed that the flow rate near the ceiling joists was zero, and maximized near the region above the soffit vents.  There are a lot of studies on the net free area requirement, but not much on the volume flow rate as a function of clime zone, roof pitch, surface material, etc.

Thoughts?  $0.01 per article linked*!

* Paid in delayed carbon credits.

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  1. Expert Member

    There are simulated results, but I was looking for actual field-collected data. Here's an interesting read for when there's more time available.

  2. user-5946022 | | #2

    There is a video out there somewhere where one of the building scientists does a simulation using smoke with different pitches. I think it might be a Joe L. video...

    1. Expert Member

      I think I've seen the one you're referencing, and if I remember correctly, they were comparing different ridge cap systems for their air flow.

  3. plumb_bob | | #4

    With all of the variables (roof construction, pitch, soffit type/size/openings, ridge vent type, wall type, wall aspect, etc etc) I feel it would be difficult to design a test that will give applicable real world results.

  4. Jon_R | | #5

    +1 on it being difficult for usable results. Wind (which they ignored in the simulation) is the primary driver.

    1. Expert Member

      I was thinking that while I read it. I suppose there's still some utility in knowing about the stratification layers, and how it changes with net free vent area.

      1. Jon_R | | #7

        With turbulent flow and outside air being cooler, I'd expect lots of mixing of the airflow as it moves towards the ridge.

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